1. Field of the Invention
The present invention relates to a gas discharge lamp, more particularly, to a cold cathode fluorescent lamp (CCFL) with a double-tube construction.
2. Description of the Related Art
Cold cathode fluorescent lamps (CCFL) have been widely used in a variety of fields such as liquid crystal displays, scanners, automobile instrument boards, small sized advertising neon signs and picture frame displays because of high luminous intensity, uniform luminous emittance, small-diameter tube and being made in various shapes. Generally, they are a novel miniature high brightness source used as a backlight for the above-mentioned products.
The working voltage of a CCFL depends mainly on the construction and material thereof (e.g., tube diameter, tube length, gas pressure inside, electrode material and construction, process for making the CCFL) as well as the requirements of starting circuit. Therefore, the output power of the CCFL won""t vary much as the working voltage increases once it is made. And the output power of the CCFL increases (i.e., an increase in the brightness) as the increase in current, which leads to an increase In the temperature of both electrodes, thereby raising the working temperature of the whole CCFL. If a part of the CCFL is affected by the environment in order to lower the temperature, the brightness of the corresponding portion will be dimmed, thereby resulting in non-uniform brightness of the CCFL. In order to solve this problem, a CCFL with a double-tube construction commercially available (FIG. 1), which comprises an inner fluorescent tube 3, electrodes 1 disposed at both ends of the inner fluorescent tube 3, a layer of fluorescence 5 coated on the internal wall surface of the inner fluorescent tube 3 and a gas 6 filled inside the inner fluorescent tube 3, characterized in that a transparent glass tube 2 is sheathed on the outside of the inner fluorescent tube 3, the space 4 therebetween is either evacuated or filled with a pressured gas, and the end 7 of the outer glass tube 2 is connected in a sealed manner with the end of the inner fluorescent tube 3.
As shown in FIG. 1, at the time that the CCFL is in operation, the inner fluorescent tube 3 is not almost affected by a change in the external temperature and environmental conditions due to that it is separated by the outer glass tube 2, thereby resulting in uniform brightness and stable luminous emission. Even though the environmental temperature is rather low, the inner fluorescent tube 3 can start and reach the required brightness within a very short period of time.
However, in the CCFL shown in FIG. 1, both ends of the inner fluorescent tube 3 are fully embedded into both ends of the outer glass tube 2, i.e., the ends of double tubes are integrally joined. When the environmental temperature becomes lower, the temperature difference between the tubes may reach over 100 degrees Celsius. The stress produced by the temperature difference therebetween may easily cause a break at the sealing ends so that the CCFL becomes useless. Therefore, this CCFL has inherent disadvantages which significantly limits its application prospects in various environments.
The present invention has been made to solve the above-mentioned technical problems and to overcome disadvantages of the related art. Accordingly, the object of the present invention is to provide a CCFL capable of operating safely and reliably suitably for using in various environments.
According to this invention, a CCFL is provided which comprises an inner fluorescent tube and an outer glass tube sheathed on the outside of the inner fluorescent tube, both of which are separately disposed and there is a space therebetween. Said CCFL also comprises electrodes sealed at the ends of the said inner fluorescent tube and the said outer glass tube.
According to the CCFL of this invention, the external surfaces of the ends of said inner fluorescent tube are connected in a seal manner with the internal surfaces of the ends of said outer glass tube.
According to the CCFL of this invention, the internal surfaces of the ends of said outer glass come into contact with the curved rounded portion of the external surfaces of the ends of said inner fluorescent tube.
According to the CCFL of this invention, the internal surfaces of the ends of said outer glass tube are not in contact with the external surfaces of the ends of the said inner fluorescent tube.
According to the CCFL of this invention, an expandable portion is built on at least one electrode located between the ends of said inner and outer tubes.
The CCFL in accordance with the present invention is provided with the double-tube construction. Due to using such a construction, the inner fluorescent tube is not almost affected by a change in the environmental temperature. Also, as the inner fluorescent tube and the outer glass tube are fully and separately disposed, the ends of double tubes are not integrally joined, thereby reducing significantly the rate of breakage due to a great temperature difference between the ends of double tubes. Furthermore, an expandable portion, which is built on the electrodes sealed between the ends of the inner fluorescent tube and the outer glass tube, can absorb completely the stress caused by the temperature difference therebetween, thereby eliminating breaking of the CCFL.